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JavaAdvanced#puzzle#collections

What is the output? List<Integer> list = new ArrayList<>(); list.add(1); list.add(2); for (Integer i : list) { if (i == 1) list.remove(i); }

Modifying a List while iterating with a for-each loop (which uses an Iterator internally) throws ConcurrentModificationException, since ArrayList's iterator is fail-fast and detects the structural modification.

Example
List<Integer> list = new ArrayList<>(List.of(1, 2));
for (Integer i : list) {
  if (i == 1) list.remove(i); // throws ConcurrentModificationException
}

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